A Brønsted-Ligand-Based Iron Complex as a Molecular Switch with Five Accessible States.

A mononuclear Fe complex, prepared with a Brønsted diacid ligand, H L (H L=2-[5-phenyl-1H-pyrazole-3-yl] 6-benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, Fe (H L) (1 ), exhibits abrupt spin transition at T =258 K, and treatment with base yields a deprotonated analogue [Fe (HL) ] (1 ), which shows gradual SCO above 350 K. A range of Fe analogues were also characterized. Fe (HL)(H L)Cl (1 ) has an S=5/2 spin state, while the deprotonated complexes [Fe (L)(HL)], (1 ), and (TEA)[Fe (L) ], (1 ) exist in the low-spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid-state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.